Each Missile Tank only increases Samus' total by 1. While that initially sounds rather meager, remember that Samus can fully replenish her payload at any time by using the Concentration technique. A whopping 70 Missile Tanks are hidden throughout the Bottle Ship. This gives Samus' arsenal a maximum of 80 when added to the initial 10.
Samus' upgrades | Missile locations | Energy Tank locations
Energy Part locations | Accel Charge locations | E-Recovery Tank locations